Factual error: When Frank knocks out the Umpire and begins to change into his clothes, the umpire is wearing white underwear. All umpires wear black in case the seat of their pants rip so the game can continue.
Continuity mistake: When Frank is fishing the pen out of the fish tank and the fish gets attached to his nose, there is visible blood. When the camera cuts back to Frank, the blood is gone. (00:20:35)
Factual error: At the onset of the baseball game, take a look at the Angel's catcher as Mariner Jay Johnstone steps up to bat. You will see he is wearing just a standard baseball hat under his catcher's helmet. MLB rules requires some form of certified (protective inside and out) plastic helmet for catchers, and thus this would not be allowed. (01:07:35)
Visible crew/equipment: At the end of the film when the guy in the wheelchair is rolling down the stairs you can see a burst of gas (presumably from a CO2 cylinder) fire the man up. (01:18:20)
Visible crew/equipment: In the opening sequence, the camera's shadow on which the car roof and siren are mounted is visible as it enters the house. (00:04:00)
Continuity mistake: When Frank is at the party when the queen arrives, before the horns blow in his ears he's about 1 foot away from them, but the close up of him shows his ears even with the end of the horns. (00:56:50)
Continuity mistake: As Frank and Jane head back to her apartment, a woman passes them by with her dog. When the camera cuts to Frank several times in the scene, you'll notice that the woman and the dog are in several different places and end up turning the corner twice. (00:42:25)
Continuity mistake: When Nordberg is in the hospital for the first time, Frank comes. Before he walks in, Nordberg's hand is on his side, and when Frank finally walks in, his hand is on his stomach. (00:10:05)
Continuity mistake: When Drebbin is riding in the back seat of the "driver's ed." car, the gear shift stays in the same position even when the car is reversing. (00:32:45)
Continuity mistake: When cutting into the cake with the queen's portrait on it, the waiter slices across the inner corner of the queen's right eye. However, the two pieces of cake served to Jane and Frank both have complete and uncut eyes. (00:54:25 - 00:54:55)
Continuity mistake: In Ludwig's office, the position of the tissue papers changes between shots. (00:21:20)
Other mistake: Pay attention to one of the early innings of the baseball game as one of the Angels' players hits a home run. Though probably intentionally set up this way as to avoid any problems of "logo patents" and such with the real-life MLB California Angels team, he is wearing a helmet without the team logo on it. Also, it is at Dodger Stadium. (01:07:35)
Audio problem: Right at the scene where Reggie Jackson points the gun at the Queen, Drebin fires a shot off at him from his watch. It goes astray, hitting some fat lady, and we hear him saying "whoops!" but his lips don't move. (01:16:10)
Continuity mistake: When Nordberg stumbles out the door on the ship and about to fall over the rail you see him going out the cabin crouching with his head first. When it cuts to a wider shot from outside you see him walking calmly against the rail before falling over. (00:07:00)
Other mistake: While Frank Drebbin is saying the words "dew glistening on the grass" you can clearly see a woman in the audience at the bottom of the screen repeating every word he says. (01:19:55)
Continuity mistake: When Frank has his speech and says "No matter how silly the idea of having a queen..." you see the mayor turning her head to the right. When it cuts she is looking to her left with a rather shocked face. (00:13:55)
Continuity mistake: When Jane is sucking on Frank's finger, you see Jane holding his finger. When it cuts, she isn't holding his finger. (00:36:40)
Continuity mistake: In the beginning of the movie when the guy with the long beard starts to talk you see Frank standing disguised by the table. When it cuts Frank is moving towards the table. (00:01:25)
Continuity mistake: When Frank sets Lugwig's place on fire and goes out the window, you see him about five or six windows away from the corner. Although in the next shot of him, he's going around the corner. (00:46:45)
Answer: Because Nordberg was working undercover in Ludwig's organization; not knowing this, the snitch would only see him working with Ludwig's crew, and would thus believe he was dirty.